1. Field of the Invention
The present invention relates to a GT-cut quartz crystal resonator.
2. Description of the Related Art
Crystal resonators for use as a reference source for a frequency or time are classified into several types of “cut” depending on a crystallographic orientation obtained when a crystal plate or crystal blank, i.e., a vibrating plate constituting a crystal resonator, is cut out from a single crystal of quartz (see, for example, Hirofumi Kawashima, Koichi Hirama, Naoya Saito, and Mitsuaki Koyama, “Quartz Resonators and Devices”, Transactions of the Institute of Electronics, Information and Communication Engineers C-I, Vol. J82-C-I, No. 12, pp. 667 to 682, December 1999). An AT-cut and an SC-cut have been widely known as examples of such a “cut”. Among them, a GT-cut crystal blank has an excellent frequency-temperature characteristic and shows an extremely small change in resonance frequency when the ambient temperature is changed. For this reason, the GT-cut crystal blank is expected to be applied to a crystal oscillator with high accuracy and high stability, for example. In addition, the GT-cut crystal resonator has advantages that it can be configured in a small size even when the resonance frequency thereof is low.
As widely known, three crystal axes of an X-axis, a Y-axis, and a Z-axis are crystallographically defined in quartz. A crystal plate cut out along a plane orthogonal to the Y-axis is called a “Y-plate”. The plane orthogonal to the Y-axis is a surface parallel to the X-axis and the Z-axis. A GT-cut crystal plate is of a quartz plate formed such that the Y-plate is rotated around the X-axis by +51.5° (i.e., φ=+51.5°) and the plate is rotated within the plane of the plate by +45° (i.e., θ=+45°). Angles “φ” and “θ” are parameters generally used to specify the cut orientation of quartz. FIG. 1 illustrates cut orientation 12 obtained when a GT-cut crystal plate is cut out from a single crystal quartz, i.e., raw stone 11. For reference, FIG. 1 also illustrates cut orientations of typical cuts other than the GT-cut. In order to specify the orientations within the GT-cut crystal plate, axes obtained by allowing the X-axis, the Y-axis, and the Z-axis to rotate around the X-axis by +51.5° are respectively defined as an X′-axis, a Y′-axis, and a Z′-axis. Since the X′-axis is obtained by allowing the X-axis to rotate about the X-axis, the X′-axis is identical with the X-axis. Axes obtained by allowing the X′-axis and the Z′-axis to rotate around the Y′-axis by 45° in the direction from the Z′-axis to the X′-axis are respectively defined as an X″-axis and a Z″-axis.
A vibration mode in the GT-cut crystal plate will now be described. As illustrated in FIG. 2, a vibration mode in GT-cut crystal plate 21 is a combined vibration mode of a longitudinal vibration mode in an X″-axis direction and a longitudinal vibration mode in a Z″-axis direction. Both of the longitudinal vibration modes in the X″-axis direction and the Z″-axis direction are a length-extensional vibration mode. Vibration modes obtained by combining these two longitudinal vibration modes are also called “a width-length extensional coupling vibration mode”. In the figures, length-extensional vibration directions are indicated by arrows, and an outline displaced by vibration is indicated by dotted lines. In this case, however, the displaced outline is illustrated as an outline with a much larger displacement than an actual displacement in crystal plate 21, for convenience of illustration. Because of such a coupled vibration mode of the two longitudinal vibration modes, the GT-cut crystal plate of the related art is formed in a rectangular shape or a square shape with one pair of sides in parallel with the X″-axis and the other pair of sides in parallel with the Z″-axis to use as a vibrating plate, i.e., a crystal blank, in the crystal resonator. Both principal surfaces of the crystal plate are provided with excitation electrodes for exciting the crystal plate serving as a vibrating plate.
In the case of using a GT-cut crystal plate as a vibrating plate, i.e., crystal blank, constituting a crystal resonator, it is necessary to hold the crystal plate within a container so as not to contact a wall surface or the like of the container of the crystal resonator. In view of this, a technique is proposed in which a main portion of a vibrating plate i.e., a vibration part, and a support portion for supporting the vibration part are integrally formed from a plate-shaped member a quartz crystal by using photolithography technique (see, JP-9-246898A; and Hirofumi Kawashima, Osamu Ochiai, Akihito Kudo, and Atsunobu Nakajima, “Miniaturized GT-Cut Quartz Resonators” The Horological Institute of Japan, Vol. 104, pp. 36-48, 1983). In this case, as illustrated in FIG. 3, support portions 22 are connected to positions of middle points on a pair of opposed sides in the main portion of a rectangular shape in crystal plate 21 serving as a vibrating plate. The shape of each support portion 22 is designed by using a finite element method or the like so that a resonance frequency of the vibration part itself is substantially the same as a resonance frequency of the entire resonance system including support portions 22.
Note that the vibration mode of the quartz crystal plate is varied depending on the type of cut. For example, in the case of the AT-cut crystal plate which has been widely used, the vibration mode is a thickness-shear vibration mode, and the resonance frequency is determined only by the thickness. For this reason, the planar shape of the AT-cut crystal plate can be arbitrarily set. For example, as disclosed in JP-2007-158486A, the planar shape may be a circular or elliptical shape. In addition, a crystal blank of the AT-cut may be supported at a position corresponding to a stationary point of a thickness-shear vibration.
In the case of a GT-cut crystal blank, however, the vibration mode is the width-length extensional coupling vibration mode, which means that the resonance frequency changes depending on the planar shape and size of the width, length, and the like of the crystal blank, and vibrations in two vibration modes coupled together must be reliably generated. Therefore, it is impossible to arbitrarily set the planar shape and to arrange the support portion at an arbitrary position. In particular, there is generally no stationary point of a vibration displacement on an outer peripheral portion of a rectangular GT-cut crystal plate.
As described above, the GT-cut crystal plate has an excellent frequency-temperature characteristic, and is suitable for constructing a crystal oscillator with high stability and high accuracy. However, the GT-cut crystal blank of the related art has a rectangular shape, and the support portions are connected to the middle points on a pair of opposed sides. Since the vibration mode of the GT-cut crystal plate is the width-length extensional coupling vibration mode, the crystal plate is vibrated and displaced at the connected positions of the support portions, and the provision of the support portions may thus hinder vibration of the crystal plate. Attempts have been made to design each support portion using a finite element method so as to be formed in a shape that has no adverse effect on the vibration of the crystal plate. However, it is difficult to manufacture such support portions, because each support portion has a complicated shape. Further, the size of the support portion itself cannot be ignored in comparison to the size of the main portion of the vibrating plate. Therefore, a variation in dimensions of each support portion greatly affects the vibration characteristics of the crystal plate, and inhibits miniaturization of the crystal resonator.